Olefin polymerization process

ABSTRACT

This invention relates to a process for the preparation of olefin polymers, preferably a multi-stage process, in which hydrogen is used to control the molecular weight of the olefin polymer produced in a continuous reactor, in particular a process wherein a metallocene or other single site catalyst is present in the polymerization reaction mixture, as well as to olefin polymers produced thereby.

This invention relates to a process for the preparation of olefinpolymers, preferably a multi-stage process, in which hydrogen is used tocontrol the molecular weight of the olefin polymer produced in acontinuous reactor, in particular a process wherein a metallocene orother single site catalyst is present in the polymerization reactionmixture, as well as to olefin polymers produced thereby.

Such preparation of polymers is effected by the use of a variety ofcatalyst systems, e.g. Ziegler-Natta catalysts, metallocene catalysts,and chromocene-silica catalysts. Typically such polymerisation may beperformed in gas phase, slurry or solution phase continuous reactors. Itis furthermore known to use hydrogen in polymerisation reactors in orderto produce polymers of the desired molecular weight. Such molecularweight control is effected through control of the concentration ofhydrogen. We have now surprisingly found that such molecular weightcontrol may be performed more efficiently through control of theconsumption of hydrogen.

Viewed from one aspect the invention provides a process for olefinpolymerization, preferably for the production of an ethylene orpropylene homo or copolymer, in particular for the preparation ofethylene copolymers, comprising polymerising at least one α-olefin in acontinuous reactor in the presence of hydrogen and an olefinpolymerization catalyst, preferably a catalyst comprising a metalloceneor other single site catalyst, the hydrogen consumption rate beingcontrolled during the polymerization whereby to control the molecularweight of the polymer product at the desired value.

Viewed from an alternative aspect the invention provides a process forolefin polymerization, preferably for the production of an ethylene orpropylene homo or copolymer, in particular for the preparation ofethylene copolymers, comprising polymerising at least one α-olefin in acontinuous, mixed reactor in the presence of hydrogen and an olefinpolymerization catalyst, preferably a metallocene or other single sitecatalyst, control of the molecular weight of polyolefin produced beingeffected by controlling the ratio between

A. The rate of hydrogen consumption in the reactor system from a massbalance

B. The rate of production of polymer in the reactor system.

The term molecular weight is to be understood as relating to anymolecular weight parameter of the polymer such as weight average MW,MFR, melt index (MFR₂), high load melt index (MFR₂₁), melt viscosity,intrinsic viscosity, viscosity average MW etc.

Viewed from another aspect the invention provides a process for olefinpolymerization, preferably for the production of an ethylene orpropylene homo or copolymer, in particular for the preparation ofethylene copolymers, which process comprises at least two continuouspolymerization stages, a relatively earlier of said stages comprisingpolymerizing at least one α-olefin in the presence of hydrogen and anolefin polymerization catalyst whereby to produce a first polymerizationproduct, and a relatively later of said stages comprising polymerizingsaid at least one α-olefin in the presence of an olefin polymerizationcatalyst whereby to yield a polymerization product having a lower MFR₂than said first polymerization product, wherein the hydrogen consumptionrate is controlled in said relatively early stage whereby to control themolecular weight of said first product.

It is preferred to use catalysts that are responsive to hydrogen: thussuch a catalyst system might comprise a combination of metallocene andchromium catalysts especially where the chromium catalyst is in form ofchromium oxide, preferably on particulate supports, especially with bothloaded together on the same support particles.

The polymer product of the single stage process of the invention if usedwith a single site catalyst will have a relatively narrow molecularweight distribution (e.g. a low MFR₂₁/MFR₂ ratio) and thus may besuitable for rotomoulding, injection moulding or production of LLDPEfilm.

Alternatively where the invention is used in a multistage process, orwhere a catalyst system having catalyst sites very responsive tohydrogen concentration and sites less responsive or non-responsive tohydrogen concentration is used, the polymerization product will have abimodal or multimodal, ie. broad, molecular weight distribution and maybe suitable for blow moulding, film, pipe, wire, fibre or cable. Byresponsive to hydrogen concentration is meant a catalyst for which themolecular weight of the polymer product is altered if the hydrogenconcentration used in the reaction mixture is varied, ie. a hydrogenconsuming catalyst. Typically metallocene catalysts are more hydrogenresponsive than Ziegler or chromium catalysts: thus such a catalystsystem might comprise a metallocene catalyst alone or a combination ofmetallocene and chromium catalysts, preferably on particulate supports,especially with both loaded together on the same support particles.

The process of the invention may optionally comprise: furtherpolymerisation stages before or after the hydrogen controlled stage,e.g. to produce a heterophasic polymer; drying steps; blending of thepolymer product with one or more further materials, e.g. furtherpolymers, antioxidants, radiation (e.g. UV-light) stabilizers,antistatic agents, fillers, plasticizers, carbon black, colors, etc.;granulation, extrusion and pelletization; etc.

Viewed from further aspects the invention provides an olefin polymerproduced by a process according to the invention as well as the use ofsuch polymers for the production of moulded articles, fibres, pipes,films, blow moulded, injection moulded and rotomoulded articles andproducts for wire and cable applications.

The process of the invention is one for the polymerization of α-olefins,in particular C₂₋₁₀ α-olefins, more particularly ethylene and propylene,especially ethylene. The polymer product of each polymerization stagemay be a homopolymer or a copolymer (which term is used to includepolymers deriving from two or more monomer species). Where the productis a copolymer, preferably at least 50% by weight of the polymer derivesfrom a C₂₋₁₀ α-olefin monomer, more particularly from a C₂₋₄ α-olefinmonomer, preferably ethylene or propylene. The other monomer(s) may beany monomers capable of copolymerization with the olefin monomer,preferably mono or polyunsaturated C₂₋₂₀ compounds, in particularmonoenes or dienes, especially C₂₋₁₀ α-olefins such as ethene, propene,but-l-ene, pent-l-ene, hex-l-ene, oct-l-ene or mixtures thereof. Bulkycomonomers, e.g. styrene or norbornene may also be used. Generally, thepolymer produced in the polymerization stages will comprise the sameα-olefin monomer, e.g. as the sole monomer or as the comonomer fromwhich at least 50%, preferably 60 to 99.8% of the copolymer derives.Thus the polymer product will preferably be an ethylene homopolymer, anethylene copolymer, a propylene homopolymer or a propylene copolymer.

If several reactors are used, the catalysts used in the differentpolymerization stages may be the same or different; however the use ofthe same catalyst is preferred. The catalysts employed may be anycatalyst capable of catalysing olefin polymerization and consuminghydrogen, e.g. Ziegler catalysts (e.g. Ziegler Natta catalysts),chromocene/silica catalysts, metallocene (ie. η-ligand complexedmetals), etc. What is required is that the catalyst which is used in thehydrogen controlled stage be one which depletes the reaction mixture ofhydrogen. It should preferably be a catalyst which uses up hydrogen morerapidly than the conventional Ziegler Natta or non-metallocene chromiumcatalysts. In this regard it is particularly preferred to use singlesite catalysts such as the catalytically effective metal:η-ligandcomplexes, ie. complexes in which the metal is complexed by the extendedΠ-orbital system of an organic ligand. Metallocenes are an example ofcomplexes in which a metal is complexed by two η-ligands—in the presentinvention metal:η-ligand complexes may be used where the metal iscomplexed by one, two or more η-ligands. The use of twin η-ligandmetallocenes and single η-ligand “half metallocenes” (e.g. thosedeveloped by Dow) is particularly preferred. However the termmetallocene as used herein is used to refer to all such catalyticallyactive complexes containing one or more η-ligands. The metal in suchcomplexes is preferably a group 4, 5, 6, 7 or 8 metal or a lanthanide oractinide, especially a group 4, 5 or 6 metal, particularly Zr, Hf or Ti.The η-ligand preferably comprises a cyclopentadienyl ring, optionallywith a ring carbon replaced by a heteroatom (e.g. N, B, S or P),optionally substituted by pendant or fused ring substituents andoptionally linked by bridge (e.g. a 1 to 4 atom bridge such as (CH₂)₂,C(CH₃)₂ or Si(CH₃)₂) to a further optionally substituted homo orheterocyclic cyclopentadienyl ring. The ring substituents may forexample be halo atoms or alkyl groups optionally with carbons replacedby heteroatoms such as O, N and Si, especially Si and O and optionallysubstituted by mono or polycyclic groups such as phenyl or naphthylgroups. Examples of such homo or heterocyclic cyclopentadienyl ligandsare well known from the scientific and patent literature, e.g. from thepublished patent applications of Hoechst, Montell, Borealis, Exxon, andDow, for example EP-A-416815, WO96/04290, EP-A-485821, EP-A-485823, U.S.Pat. No. 5,276,208 and US-A-5145819.

Thus the η-bonding ligand may for example be of formula I

CpY_(m)  (I)

where Cp is an unsubstituted, mono-substituted or polysubstituted homoor heterocyclic cyclopentadienyl, indenyl, tetrahydroindenyl, fluorenyl,benzindenyl, cyclopenta[l]phenanthrenyl, azulenyl, or octahydrofluorenylligand; m is zero or an integer having a value of 1, 2, 3, 4 or 5; andwhere present each Y which may be the same or different is a substituentattached to the cyclopentadienyl ring moiety of Cp and selected fromhalogen atoms, and alkyl, alkenyl, aryl, aralkyl, alkoxy, alkylthio,alkylamino, (alkyl)₂P, alkylsilyloxy, alkylgermyloxy, acyl and acyloxygroups or one Y comprises an atom or group providing an atom chaincomprising 1 to 4 atoms selected from C, O, S, N, Si and P, especially Cand Si (e.g. an ethylene group) to a second unsubstituted,mono-substituted or polysubstituted homo or heterocycliccyclopentadienyl, indenyl, tetrahydroindenyl, fluorenyl oroctahydrofluorenyl ligand group.

In the η-bonding ligands of formula I, the rings fused to the homo orhetero cyclopentadienyl rings may themselves be optionally substitutede.g. by halogen atoms or groups containing 1 to 10 carbon atoms.

Many examples of such η-bonding ligands and their synthesis are knownfrom the literature, see for example: Möhring et al. J. Organomet. Chem479:1-29 (1994), Brintzinger et al. Angew. Chem. Int. Ed. Engl.34:1143-1170 (1995).

Examples of suitable η-bonding ligands include the following:

cyclopentadienyl, indenyl, fluorenyl, pentamethyl-cyclopentadienyl,methyl-cyclopentadienyl, 1,3-di-methyl-cyclopentadienyl,i-propyl-cyclopentadienyl, 1,3-di-i-propyl-cyclopentadienyl,n-butyl-cyclopentadienyl, 1,3-di-n-butyl-cyclopentadienyl,t-butyl-cyclopentadienyl, 1,3-di-t-butyl-cyclopentadienyl,trimethylsilyl-cyclopentadienyl, 1,3-di-trimethylsilyl-cyclopentadienyl,benzyl-cyclopentadienyl, 1,3-di-benzyl-cyclopentadienyl,phenyl-cyclopentadienyl, 1,3-di-phenyl-cyclopentadienyl,naphthyl-cyclopentadienyl, 1,3-di-naphthyl-cyclopentadienyl,1-methyl-indenyl, 1,3,4-tri-methyl-cyclopentadienyl, 1-i-propyl-indenyl,1,3,4-tri-i-propyl-cyclopentadienyl, 1-n-butyl-indenyl,1,3,4-tri-n-butyl-cyclopentadienyl, 1-t-butyl-indenyl,1,3,4-tri-t-butyl-cyclopentadienyl, 1-trimethylsilyl-indenyl,1,3,4-tri-trimethylsilyl-cyclopentadienyl, 1-benzyl-indenyl,1,3,4-tri-benzyl-cyclopentadienyl, 1-phenyl-indenyl,1,3,4-tri-phenyl-cyclopentadienyl, 1-naphthyl-indeny,1,3,4-tri-naphthyl-cyclopentadienyl, 1,4-di-methyl-indenyl,1,4-di-i-propyl-indenyl, 1,4-di-n-butyl-indenyl, 1,4-di-t-butyl-indenyl,1,4-di-trimethylsilyl-indenyl, 1,4-di-benzyl-indenyl,1,4-di-phenyl-indenyl, 1,4-di-naphthyl-indenyl, methyl-fluorenyl,i-propyl-fluorenyl, n-butyl-fluorenyl, t-butyl-fluorenyl,trimethylsilyl-fluorenyl, benzyl-fluorenyl, phenyl-fluorenyl,naphthyl-fluorenyl, 5,8-di-methyl-fluorenyl, 5,8-di-i-propyl-fluorenyl,5,8-di-n-butyl-fluorenyl, 5,8-di-t-butyl-fluorenyl,5,8-di-trimethylsilyl-fluorenyl, 5,8-di-benzyl-fluorenyl,5,8-di-phenyl-fluorenyl and 5,8-di-naphthyl-fluorenyl.

Besides the η-ligand, the catalyst complex used according to theinvention may include other ligands; typically these may be halide,hydride, alkyl, aryl, alkoxy, aryloxy, amide, carbamide or other twoelectron donor groups.

In a preferred embodiment the catalyst composition comprises twoη-ligand catalysts, most preferably a combination of an unbridged (alkylsubstituted) metallocene with a substituted silicon bridged bis(indenyl)zirconium dichloride catalyst, the latter preferably being substitutedon the 1 and 4 positions.

Where a Ziegler Natta catalyst is used in the later polymerizationstage, this may be any convenient Ziegler Natta catalyst, e.g. a group 4metal chloride (e.g. titanium chloride) associated with MgCl₂, MgO orSiO₂ (see also PCT/SE96/01652).

The catalyst systems used may of course involve co-catalysts or catalystactivators and in this regard any appropriate co-catalyst or activatormay be used. Thus for η-ligand complexes, aluminoxane or boron compoundcocatalysts may be used. It is envisaged however that the use of extraadded cocatalyst may not be required.

Preferred aluminoxanes include C₁₋₁₀ alkyl aluminoxanes, in particularmethyl aluminoxane (MAO) and aluminoxanes in which the alkyl groupscomprise isobutyl groups optionally together with methyl groups. Suchaluminoxanes may be used as the sole co-catalyst or alternatively may beused together with other co-catalysts. Thus besides or in addition toaluminoxanes other cation complex forming catalyst activators may beused. In this regard mention may be made of the silver and boroncompounds known in the art. What is required of such activators is thatthey should react with the η-liganded complex to yield an organometalliccation and a non-coordinating anion (see for example the discussion onnon-coordinating anions J in EP-A-617052 (Asahi)).

Aluminoxane co-catalysts are described by Hoechst in WO 94/28034. Theseare oligomers having up to 40, preferably 3 to 20, [—Al(R″)O]— repeatunits (where RI″ is hydrogen, C₁₋₁₀ alkyl (preferably methyl and/orisobutyl) or C₆₋₁₈ aryl or mixtures thereof) and probably have cage likestructures.

It is preferred that no separate feed of liquid cocatalyst is used assuch a feed may disturb chain transfer reactions.

It is particularly desirable that the catalytic sites or theirprecursors be supported on a solid substrate to obtain particulatecatalysts for use in such polymerization reactions. Such substrates arepreferably porous particulates, e.g. inorganic oxides such as silica,alumina, silica-alumina or zirconia, inorganic halides such as magnesiumchloride, or porous polymer particles, e.g. acrylate polymer particlesor styrene-divinylbenzene polymer particles. Weight average medianparticle sizes are preferably in the range 10 to 60 μm and porositiesare preferably in the range 1 to 3 mL/g. The complex may be loaded ontothe support before, or more preferably after it has been reacted with aco-catalyst. Desirably, inorganic supports are heat treated (calcined)before being loaded with the complex.

The catalyst system used may have more than one active catalytic site.Preferably these are uniformly distributed over the support (carrier)particles, especially with each type of site being uniformly distributedon the same particles. Preferably one of the types of active sites ismore hydrogen consuming than the other, e.g. the support particles couldbe loaded with a metallocene and a chromium catalyst or a metalloceneand a Ziegler catalyst or two metallocene catalysts of differenthydrogen consuming ability.

The processes of the invention may be carried out in a single reactor orin a series of two or more reactors. Each polymerization stage may beeffected using conventional procedures, e.g. as a slurry, gas phase,solution or high pressure polymerization. Slurry polymerisation includespolymerisation at slightly supercritical conditions. Mixed gas phase andslurry reactors are preferred. Slurry polymerization (e.g. bulkpolymerization) is preferably effected, e.g. in a tank reactor or morepreferably a loop reactor. The polymerization process may use a seriesof two or more reactors, preferably loop and/or gas phase reactors, e.g.a combination of loop and loop, gas phase and gas phase or mostpreferably loop and gas phase reactors. In slurry reactors, if a majormonomer is propylene this may also function as a solvent/diluent as wellas a reagent. If the major monomer is ethylene, a non-polymerizableorganic compound, e.g. a C₃₋₁₀ alkane, for example propane or isobutane,may be used as a diluent. Where this is done, the volatile non-reactedor non-reactive materials will desirably be recovered and reused.

Most preferably the process of the invention is effected, after aninitial prepolymerisation stage, in a slurry loop reactor followed by agas phase fluidised reactor, optionally employing a further gas phasereactor after the first gas phase. In any of these reactors, thehydrogen control of molecular weight may take place. The reactor used inthe polymerization process of the invention should be continuous. Thisshould be understood also to include the case when feeds to and flowsfrom the reactor may be intermittent if the time constant of theintermittent flows are shorter than the average residence time in thereactor.

Such multi-reactor processes may be used for example for the productionof heterophasic (impact resistant) polypropylene where in one stagemainly polyethylene may be produced using for example a metallocenecatalyst and hydrogen to control molecular weight.

Typical reaction conditions for loop and gas phase reactors are:loop—temperature 60-110° C., pressure 30-70 bar, mean residence time30-80 minutes; and gas phase—temperature 60-110° C. (preferably 80-105°C.), pressure 10-25 bar, mean residence time 20-300 minutes. Wherehydrogen is used to control molecular weight/MFR₂, the hydrogen partialpressure will typically be 0.0001 to 0.5 bar.

The final polymer product of the process of the invention willpreferably have a MFR₂₁ of about 1 to an MFR₂ of about 100, a weightaverage molecular weight (Mw) of 30000 to 500000, a melting point of100-1650° C. (e.g. 100-136° C. for polyethylenes and 120 to 165° C. forpolypropylenes) and a crystallinity of 20 to 70%. The polymer product ofa hydrogen consumption-controlled stage of a multistage processaccording to the invention is preferably from MFR₂₁ about 0.01 to MFR₂about 5000. (i.e. if only this reaction stage were performed using thesame reaction conditions then these would be the MFR values).

This polymer can be formulated together with conventional additives,e.g. antioxidants, UV-stabilizers, colors, fillers, plasticizers, etc.and can be used for fibre or film extrusion or for raffia, or for pipes,or for cable or wire applications or for moulding, e.g. injectionmoulding, blow moulding, rotational moulding, etc., using conventionalmoulding and extrusion equipment.

In the process of the invention, control over the molecular weight ofthe polymer produced in a stage involving use of hydrogen and a hydrogenresponsive catalyst can be readily achieved by monitoring of thehydrogen and monomer consumption, ie. for hydrogen the differencebetween hydrogen fed in and hydrogen flow out and for monomer thedifference between monomer fed in and flow out. The ratio of hydrogenconsumption to monomer consumption can be correlated well with polymermolecular weight or MFR (e.g. MFR₂) and the product molecular weight orMFR can accordingly be adjusted to the desired level using thiscorrelation and by appropriate adjustment of the hydrogen and monomerfeed rate levels. This is a novel means of molecular weight control andforms a further aspect of the invention. Viewed from this aspect theinvention provides a method of olefin polymerization in a continuousthroughput reactor, e.g. a gas phase or loop reactor, in which hydrogenand an olefin monomer are continuously introduced into said reactor andpolymer and unreacted monomer are continuously removed from saidreactor, characterised in that the ratio between the difference orpredicted difference between hydrogen fed into and flow out from thereactor and the polymer production rate or predicted production rate inthat reactor is determined or predicted and adjusted, e.g. manually,regularly or continuously, to a value within a desired range whereby tocause the polymer removed from said reactor to have a desired value of amolecular weight related parameter, e.g. MFR (ie. melt flow rate, meltindex, high load melt index etc, for example MFR₂), melt viscosity,intrinsic viscosity, weight average molecular weight, number averagemolecular weight, viscosity average molecular weight, etc.) Wherehydrogen conversion is greater than 50%, preferably where it is greaterthan 80%, the difference between hydrogen input and hydrogen output mayif desired be replaced simply by the hydrogen input value. Similarly,the difference between monomer input and output may be replaced by thepolymer production rate. The method of the invention is preferably usedif the hydrogen conversion is greater than 50%, preferably greater than80% and most preferably when conversion is greater than 90%.

In this method, the polymerization catalyst advantageously comprises anη-liganded metal as discussed herein, preferably a group 4 to 6 metal,particularly Zr, Hf or Ti and preferably Zr or Hf. The method isparticularly advantageous when the ratio of hydrogen output from tohydrogen input to the reactor is from 0 to 50:100, especially 0 to20:100. Furthermore the method is particularly suited to polymerizationprocesses in which polymer particles are formed, e.g. bulk, slurry orgas phase reactions rather than solution reactions, for exampleprocesses where the reactor temperature is less than 115° C. The methodis especially preferred for the production of ethene and propene homo-or copolymers (which latter term includes also polymers comprising threeor more comonomers).

The measurement of molecular weight (and related parameters) forpolymers from a polyolefin-producing plant is usually done in alaboratory on samples of polymer powder taken out from the process aftera reactor, often after an in-process drying step. Such measurement isresource-consuming, so usually samples are measured at intervals of manyhours. This means that if an important deviation in such parameters isdiscovered, many hours of production of the deviating product mayalready have been made. More recently, in-line measurements based onmelt viscosity are coming into use to reduce such risk. However, theseinstruments are often placed far downstream of the polymerizationprocess, so the ideal goal of getting a direct measurement of themolecular weight related parameter of the polymer being produced is notsolved. Even if such an instrument was placed close to thepolymerization process, there would still be a time lag since even thecomposition in the reactor itself would not be the same as the actualpolymer being polymerized at that instant in the reactor.

Also, since the polymer in a reactor is often a mixture of polymer madein that reactor with one or more made in previous reactors, andespecially since the previously made polymer may have been made withmuch higher molecular weight than the polymer being made in the presentreactor, in which case any polymer solution or polymer melt viscositybased molecular weight will then almost exclusively be determined by thepreviously made polymer, in which case the viable method of achievingmolecular weight control over the polymer made in the present stage, toensure consistent quality of the final polymer, is to calculate thisfrom the reactor parameters, especially from the effect of hydrogen. Soanother aspect of this invention is using the ratio of chemicalconsumption of hydrogen over the production rate in a reactor toestimate the molecular weight made in that reactor, or to provideconsistency of the product made in that reactor, or to provideconsistency of the final polymer product from the process, in theabsence of measured polymer molecular weight from that polymerizationstage being used as the basic control parameter.

In order to produce a polymer with the right directly measurablemolecular weight related parameter, the following method is usuallypresently used:

1. Based on the molecular weight goal and the previous measurements ofthe molecular weight, and the concentrations in the reactor during theprevious time (hydrogen, monomers, cocatalyst (if present)), reactortemperature and catalyst type, are used to calculate or guess afavourable value for the hydrogen concentration or the ratio betweenhydrogen concentration and monomer concentration.

2. The hydrogen feed to the reactor is controlled to reach thismentioned favourable hydrogen concentration or ratio between hydrogenand monomer, and is then maintained at this value.

3. Repeat steps 1 and 2 periodically.

As described above, step 1 constitutes the outer loop of a cascadedcontrol system of which step 2 constitutes the faster inner loop. Afurther inner loop could be in the cascaded system if a hydrogen flowmeasurement is used to control the hydrogen flow value.

A control room operator was usually required to perform step 1. Nowcomputer control is usually used. For instance, computer models maypredict behaviour of concentrations in the reactor and from thesepredictions the molecular weight. Advanced models may include use of amechanistic, kinetic approach to molecular weight control. Such anapproach is shown in: K. McAuley and J. MacGregor, AlChE Journal, Vol.37, no. 6, pages 825-835.

The control methods currently in use allow an estimation of themolecular weight of the resultant polymer directly from theconcentration of hydrogen (or ratio of hydrogen concentration to monomerconcentration) in the reactor. The methods do not require an estimate ofhydrogen consumption which depend heavily on the field of hydrogen.

It should be noted that there exist different control philosophies whichwill affect the detailed methods of exploiting the invention. Forinstance, it may be a goal to keep constant the molecular weight beingpolymerized in a reactor or leaving a reactor. The goal may also be toget consistent product out of the final reactor of a series. When doinggrade transitions, either by process parameters or by catalyst or byboth, other goals than keeping constant product may apply.

In the method of the invention the rate of chemically consumed molecularhydrogen may be found by mass balance as the difference between the rateof molecular hydrogen going into the reactor system and the sum of therates of molecular hydrogen leaving the reactor and accumulating in thereactor. A similar mass balance can be made by omitting theaccumulation, however, this is not preferred.

To achieve a true value for the chemically consumed hydrogen raterelevant to molecular weight control, corrections may have to be donefor side reactions involving hydrogen. For example hydrogen may beproduced from monomer (for example ethylene) during the course ofpolymerisation. The rate of generation of hydrogen from an additionalhydrogen source—or the rate of an additional consumption of hydrogen—maybe approximated in various ways, and used to correct abovementioned massbalance to achieve the most true value of hydrogen consumed by chaintransfer reactions. Also it should be noted that hydrogen may enter thereactor both as a controlled feed, or as non-reacted hydrogen in theproduct leaving a previous reactor, as well as by recirculation streams.

The rate of consumption of monomer is best found by a heat balance ofthe reactor or a mass (or molar) balance of monomer, or a combination ofthese. By mass or molar balance method, the production rate of polymeris the difference between monomer going into the reactor system and thesum of the rates of monomer leaving the reactor and accumulating in thereactor. This balance might be done on weight basis or molar basis. Bythe heat balance method, the rate of heat generation by polymerisationis found in essence as the difference between the sum of rates of heatremoved by the cooling system (needed for heating of feed streams to thereactor temperature), accumulating in the reactor and for heat loss, andthe sum of the rates of optional heat lost by mass at above the reactortemperature leaving the reactor system, and that generated by agitation.The polymerisation rate can then be found from the rate of heatgeneration by polymerisation, through the value of heat ofpolymerisation (heat generated per weight or mol monomer polymerised). Asimplified mass balance neglects the monomer accumulated in the reactor;however, this is not preferred.

The reactor system over which these mass and heat balances should betaken should in many cases include more than the reactor vessel itself.Thus for a fluidised gas phase reactor the optional cooling/fluidizingsystem taking gas from the reactor bed and returning it as gas orpartial condensate after cooling, is included in the reactor system. Inslurry tank reactors the optional cooling system where cooling iseffected by boiling liquid off from the slurry, then partly condensingthe gas and returning the condensed liquid and residual gas to theslurry, is also included in the reactor system.

The ratio between rate of hydrogen chemical consumption and the rate ofproduction of polymer in the reactor system can then be found. It shallof course be understood that the details of a polymerization process arecomplicated and specific arrangements of each process may need to beconsidered in detail to arrive at the exact hydrogen chemicalconsumption rate and the polymerization rate. This will be readilyachieved by the person skilled in the art.

In order to produce a polymer with the right directly measurablemolecular weight related parameter, the following method may thus beused:

1. The molecular weight goal, the molecular weight measurement duringthe last period and the chemical consumption ratio during the lastperiod, are used to calculate or estimate a favourable set point for thechemical consumption ratio. In addition, the type of catalyst andcocatalyst as well as the concentrations of reagents and reactortemperature may be used to modify this calculation to further improvethe result.

2. The hydrogen concentration or the ratio between hydrogenconcentration and ethylene concentration should be used to calculate aset point for the hydrogen fed flow.

3. The hydrogen feed flow is controlled using the hydrogen feed valve.

4. Repeat steps 1, 2 and 3 periodically.

Thus described, the hydrogen control system consists of a cascadedsystem of which step 1 constitutes the outer loop and step 2 constitutesthe inner faster loop and step 3 the innermost fastest loop.

A preferred method of employing the invention, is by using predictivecomputer control. An example of such a method is outlined below:

1. The target molecular weight parameter is set by the operator.

2. The target molecular weight parameter from step 1 is translated intoa (target) consumption ratio between hydrogen and monomer. This is doneby previous experience/knowledge about the relation between this ratioand the molecular weight parameter.

3. Kinetic models are used to calculate the consumption (reaction rate)of hydrogen and the polymerisation rate of monomer, and hence thechemical consumption ratio between hydrogen and monomer.

4. The kinetic model constants are corrected using the mass balance forhydrogen and monomer to adapt the model to the actual process behaviour.

5. Using the kinetic models along with mass balances for the componentsin the reactor system, the way hydrogen consumption is influenced byhydrogen feed, and polymerisation rate is influenced by monomer feedand/or catalyst feed is calculated This model also can be used topredict the behaviour of the process, and is an important part of thecontroller in step 6.

6. Using model based predictive control (MPC) algorithm as described inM. Hillestad, K. S. Andersen: Model predictive control for gradetransitions for a polypropylene reactor, 4th European Symposium onComputer Aided Process Engineering, Dublin, Mar. 18-30, 1994, the valuefor the molecular weight parameter can be controlled to the target foundin step 1 using hydrogen feed and/or the monomer feed as manipulatedvariables.

Another preferred way of employing the invention is by using a cascadedcontrol loop system for molecular weight control.

It should also be noted that although it is preferred to estimate amolecular weight related parameter from the ratio of hydrogen chemicalconsumption over polymer production, there are other options. Especiallyfor multistep processes, consistency of the final product may beobtained with the help of ratio of hydrogen chemical consumption overpolymer production by other means than by estimation of a molecularweight parameter, for instance by multivariate soft models.

If there is a high conversion of both the main monomer and hydrogen (forexample, above 85%), a simple, approximate version of above step 1 wouldbe to base control of molecular weight on the ratio of hydrogen feedover the polymer production rate.

For step no. 1, it is of interest to have a computer model of molecularweight versus reactor process operating data for the same purpose as thekinetic equation developed by McAuley et al. (supra).

The following is an example of development of such an equation system:

The number-average molecular weight of the polymer is the sum of chaintransfer rates divided by the propagation rate $\begin{matrix}{{{(1)\quad \frac{1}{X_{n}}} = {\frac{r_{o} + r_{h}}{r_{p}} = {\frac{r_{o}}{r_{p}} + \frac{r_{h}}{r_{p}}}}}{{(2)\quad \frac{1}{X_{n}}} = {{f\left( {c_{m1},c_{m1},c_{m2},c_{c},T,\ldots}\quad \right)} + \frac{r_{h}}{r_{p}}}}} & (1)\end{matrix}$

where f(C_(m1),C_(m1),C_(m2),C_(c),T, . . . ) is a function of reactorparameters except hydrogen.

If f(C_(m1),C_(m1),C_(m2),C_(c),T, . . . ) may be considered constant,equation (2) can be written:${(3)\quad \frac{1}{X_{n}}} = {K + \frac{r_{h}}{r_{p}}}$

where

C Concentration

K Constant

r Molar rate

T Temperature

X_(n) Number-average degree of polymerisation.

Indices:

c Cocatalyst

h Hydrogen

m1 Monomer 1

m2 Monomer 2

o Reactor parameters that are not related to hydrogen

p Propagation

Specifically:

r_(o) Rate of chain transfer reactions that are not dependent onhydrogen

r_(h) Rate of chain transfer reaction with hydrogen=Rate of hydrogenchemical consumption

r_(p) Rate of propagation of monomer=Polymerisation rate

From the number-average degree of polymerisation one can reach othermolecular weight relevant parameters. For instance, MFR is usuallyconsidered related to this as:

MFR=Const·(X_(n))^(α)  (4)

Usually a is found to be about:

α=−3.5  (5)

Equations (3), (4) and (5) together give${(6)\quad {MFR}} = {{Const}.\left\lbrack {K + \frac{r_{h}}{r_{p}}} \right\rbrack^{3\frac{1}{2}}}$

This equation gives a direct means of predicting the MFR by the ratiobetween the hydrogen chemical consumption rate and the polymerisationrate. It also provides a means to predict the MFR change from previousconditions from the change in this ratio.

Equation (6) can be transformed to${{(7)\quad \frac{r_{h}}{r_{p}}} + K} = \left\lbrack \frac{MFR}{Const} \right\rbrack^{\frac{1}{3.5}}$

This equation gives a means of establishing a set point for r_(h)/r_(p)from the desired value of MFR.

When the catalyst consumes hydrogen fast and also reduces molecularweight very fast with increasing amounts of hydrogen, then, depending onthe intended molecular weight of the product, it may happen that theanalysed hydrogen concentration will be relatively uncertain, and it mayeven be that it is below the detection limit. This makes control ofmolecular weight by hydrogen concentration very difficult/uncertain.

However, exactly the opposite happens with control based on theinvention: usually the amount of hydrogen fed can be measured ratherprecisely/reproducibly. If the conversion of hydrogen is low, the amountof hydrogen leaving the reactor can be measured rather precisely.However, if there is low conversion of hydrogen, there is very littledifference between the amount of hydrogen going in and out, and thedifference between these is not precise/reproducible. But if thehydrogen conversion is high, then the difference between hydrogen goingin and going out becomes large in comparison to the amount of hydrogengoing out. Then the difference can be calculated quite precisely.

Also, there could occur disturbances in the relationship betweenhydrogen concentration in the reactor and the molecular weight of thepolymer, due to different kinetics in the chain transfer reactionstaking place with hydrogen. This might occur if there was a change inthe reactor temperature, a change in the properties of the catalyst or achange in the mass transfer properties between the medium betweenpolymer particles and the active sites. The method of the presentinvention does not suffer from these disadvantages, especially where inEquation (3) K<<r_(h)/r_(p).

Monomer concentration used as input in present control systems is basedon monomer concentration outside polymer particles, so that disturbancesin mass transfer properties of the medium between polymer particles andthe active sites also will disturb the molecular weight control. (See T.F. McKenna et al., J. Appl. Pol. Sci., Vol 63 (1997), pages 315-322.)The method of the invention is independent of such mass transfergenerated disturbances.

In slurry loop reactors for polyethylene with settling legs, the monomerconcentration usually is measured after the settling legs, and this doesnot give a precise estimate of ethylene concentration in the actual loopbecause of all the extra conversion of ethylene taking place in thesettling legs. Since in this case, the traditional method of controlwould be based on the ratio between hydrogen and ethylene concentration,the result would be poor if the extra conversion of ethylene in thesettling legs is not very stable.

The present invention will now be described further with reference tothe following non-limiting Examples.

EXAMPLE 1 Catalyst Preparation

Porous silica powder (Sylopol 55SJ from Grace Davison) was calcined for4 hours in a flow of dry air at 600° C. The product, the catalystsupport or carrier had a pore volume of about 1.55 mL/g.

An impregnation solution was prepared by mixing under nitrogen (nBu-Cp)₂ZrCl₂ (Eurocene 5031 from Witco) 0.953 kg, MAO solution (30 wt % MAO intoluene from Albemarle SA) 92L, and toluene 25.4 kg.

86 kg of the carrier at 25° C. was placed in a steel vessel fitted witha stirrer. The impregnation solution was added over a period of 1.5hours with agitation. Agitation was continued for a further 3 hours.Over a period of 7 hours, the mixture was dried by nitrogen flow and byheating to about 45° C. A final vacuum drying was effected to yield asupported catalyst having a Zr content of 0.14 wt % and an aluminiumcontent of 11.0 wt %.

EXAMPLE 2 Polymerization

A supported catalyst was prepared analogously to Example 1 but withincreased amounts of Zr complex and MAO such that the product had a Zrcontent of 0.25 wt % and an aluminium content of 13.6 wt %. Thiscatalyst was used in a continuous 500L loop reactor for copolymerizationof ethene and hex-l-ene, operating at 85° C. and 65 bar with 1 to 2hours average residence time.

EXAMPLE 3 Polymerisation (Comparative)

A supported Ziegler Natta catalyst comprising a silica carrier, 2% byweight Ti, 1.9% by weight, 2.4% by weight Mg, Al and Cl (prepared inaccordance with Example 2 of WO95/35323) was used as the catalyst inplace of the metallocene catalyst in a one stage polymerization asdescribed in Example 2.

EXAMPLE 4 Polymerization Parameters and Product Properties

The polymerization parameters and the properties of the productsproduced in the one stage polymerizations of Examples 2 to 4 are set outbelow in Tables 1, 2 and 3 respectively.

TABLE 1 Example No. 2.1 2.2 2.3 2.4 2.5 Catalyst Feed Rate (g/hr) 21.526.0 9.0 8.0 9.0 Diluent (Propene) Feed Rate 30 25 27 34 26 (kg/hr)Ethene Feed Rate (kg/h) 30 21.5 33 29.5 19 Hex-l-ene Feed Rate (kg/h)0.92 0.8 0.9 1.3 0.55 Hydrogen Feed Rate (g/h) 0 0 1.5 2 5 ReactorProduct Ethylene (mol %) 7 8 6.2 7.5 8.0 Hydrogen (mol %) 0 0 ND ND NDSolids (wt %) 13 10 20 22 13 H₂/C₂H₄ (mol/kmol) 0 0 <2.4 <1.6 <1.9C₆H₁₂/C₂H₄ (mol/kmol) 35 33 40 50 40 % H₂ Conversion — — >87% >90% >96%MFR₂ 1.5 1.9 27 80 388 ND = not detectable

TABLE 2 Example No. 3.1 3.2 3.3 Catalyst Feed Rate (g/hr) 5.0 5.0 5.7Diluent (Propane) Feed Rate 53 52 50 (kg/hr) Ethene Feed Rate (kg/h)29.9 29.2 28.6 But-l-ene Feed Rate (kg/h) 2.4 2.5 2.4 Hydrogen Feed Rate(g/h) 32 31 33 Reactor Product Ethylene (mol %) 7.2 7.0 7.2 Hydrogen(mmol/mol total) 10.3 10.2 10.6 Solids (wt %) 18.9 18.4 18.6 H₂/C₂H₄(mol/kmol) 138 141 143 C₄H₈/C₂H₄ (mol/kmol) 325 327 309 % H₂ Conversion16 16 21

As can be seen from Tables 1 and 2, the use of the η-liganded catalyst(as would take place in the early polymerization stage of the process ofthe invention) gives higher hydrogen conversion than can be done withthe conventional Ziegler Natta catalysts.

What is claimed is:
 1. A process for olefin polymerization comprisingpolymerising at least one α-olefin in a continuous reactor in thepresence of hydrogen and an olefin polymerization catalyst, the catalystsystem having catalysts with different active catalytic sites, thecatalysts having different hydrogen consuming abilities whereby a rateof hydrogen consumption is controlled during the polymerization wherebyto control the molecular weight of a polymer product, and wherein thehydrogen conversion is greater than 80%.
 2. A process as claimed inclaim 1 wherein the polymer product is an ethylene or propylene homo orcopolymer.
 3. A process as claimed in claim 2 wherein the polymerproduct is an ethylene copolymer.
 4. A process as claimed in claim 1wherein the olefin polymerisation catalyst comprises a metallocenecatalyst.
 5. A process as claimed in claim 4 wherein the olefinpolymerisation catalyst comprises a metallocene catalyst and a chromiumcatalyst or a combination of two metallocene catalysts.
 6. A process asclaimed in claim 5 wherein the olefin polymerisation catalyst comprisesa twin η-ligand metallocene or a single η-ligand metallocene.
 7. Aprocess as claimed in claim 1 wherein the catalyst is carried on a solidsupport.
 8. A process as claimed in claim 7 wherein active catalyticsites are distributed evenly over the solid support.
 9. A process asclaimed in claim 1 wherein the process comprises at least two continuouspolymerization stages, a relatively earlier of said stages comprisingpolymerizing at least one α-olefin in the presence of hydrogen and anolefin polymerization catalyst whereby to produce a first polymerizationproduct, and a relatively later of said stages comprising polymerizingsaid at least one α-olefin in the presence of an olefin polymerizationcatalyst whereby to yield a polymerization product having a lower MFR₂than said first polymerization product, wherein the hydrogen consumptionrate is controlled in said relatively early stage whereby to control themolecular weight of said first product.
 10. A process as claimed inclaim 9 wherein the polymerisation takes place in mixed gas phase andslurry reactors.
 11. A process as claimed in claim 10 wherein a firststage takes place in a slurry loop reactor and a second stage takesplace in a gas phase fluidised reactor.
 12. A process as claimed inclaim 1 wherein hydrogen consumption rate is controlled by predictivecomputer control.
 13. A process as claimed in claim 1 wherein hydrogenconsumption rate is found by mass balance as the difference between therate of molecular hydrogen going into the reactor system and the sum ofthe rates of molecular hydrogen leaving the reactor and accumulating inthe reactor.
 14. A process as claimed in claim 1 wherein the molecularweight of the product is controlled by controlling both the hydrogenconsumption rate and the monomer consumption rate.
 15. A process asclaimed in claim 14 wherein the molecular weight of the product iscontrolled by analysis of the ratio between hydrogen consumption rateand monomer consumption raze.
 16. A process for olefin polymerizationcomprising polymerising at least one α-olefin in a continuous reactor inthe presence of hydrogen and an olefin polymerization catalyst a ratioof chemical consumption of hydrogen over a production rate in saidreactor being controlled whereby to control the consistency of a polymerproduct made in said reactor or control consistency of a final polymerproduct.
 17. A process as claimed in claim 16 wherein the hydrogenconversion is greater than 80%.